After a terrible earthquake, Satoru wakes up in a pitch black cave that him and his friends were exploring. Desperately moving through the crumbling cave with just his backpack, he expects a warm rescue once he gets out. But what awaits Satoru was an unbelievable fact: Japan has sunk underwater due to the earthquake and he is stranded on an island with no one to help him. Can Satoru learn how to survive by himself?